The present invention relates generally to coal additives for furnaces and specifically to coal additives for slag-type furnaces.
Coal is widely recognized as an inexpensive energy source for utilities. Coal-fired furnaces are used to generate steam for power production and industrial processes. Coal-fired furnaces have many different configurations and typically include a plurality of combustors. In one furnace configuration, a slag layer forms on a surface of the burner and captures the coal particles for combustion. Such a furnace will be hereafter referred to as a xe2x80x9cslag type furnace.xe2x80x9d
An example of a combustor 100 for a slag-type furnace is depicted in FIG. 1. The depicted combustor design is used in a cyclone furnace of the type manufactured by Babcock and Wilcox. Cyclone furnaces operate by maintaining a sticky or viscous layer of liquid (melted) ash (or slag) (not shown) on the inside cylindrical walls 104 of the cyclone combustion chamber 108. Coal is finely crushed (e.g., to minus xc2xc inch top size), entrained in an airstream, and blown into the combustor end 112 of the cyclone combustor or combustor 100 through coal inlet 116. Combustion air (shown as primary air 120, secondary air 124, and tertiary air 128) is injected into the combustion chamber 108 to aide in combustion of the coal. The whirling motion of the combustion air (hence the name xe2x80x9ccyclonexe2x80x9d) in the chamber 108 propels the coal forward toward the furnace walls 104 where the coal is trapped and bums in a layer of slag (not shown) coating the walls. The re-entrant throat 140 (which restricts escape of the slag from the chamber 108 via slag tap opening 144) ensures that the coal particles have a sufficient residence time in the chamber 108 for complete combustion. The slag and other combustion products exit the chamber 108 through the slag tap opening 144 at the opposite end from where the coal was introduced. The molten slag (not shown) removed from the chamber 108 flows to a hole (not shown) in the bottom of the boiler where the slag is water-quenched and recovered as a saleable byproduct. The ash composition is important to prevent the slag from freezing in the hole and causing pluggage. To melt ash into slag at normal combustion temperatures (e.g., from about 2600 to about 3000xc2x0 F.), slag-type furnaces, such as cyclones, are designed to burn coals whose ash contains high amounts of iron and low amounts of alkali and alkaline earth metals (as can be seen from FIG. 2). Iron both reduces the melting temperature of the ash and increases the slag viscosity at these temperatures due to the presence of iron aluminosilicate crystals in the melt.
High sulfur content in coal, particularly coals from the eastern United States, has allegedly caused significant environmental damage due to the formation of sulfur dioxide gas. As a result, utilities are turning to low sulfur western coals, particularly coals from the Powder River Basin, as a primary feed material. As used herein, xe2x80x9chigh sulfur coalsxe2x80x9d refer to coals having a total sulfur content of at least about 1.5 wt. % (dry basis of the coal) while xe2x80x9clow sulfur coalsxe2x80x9d refer to coals having a total sulfur content of less than about 1.5 wt. % (dry basis of the coal) and xe2x80x9chigh iron coalsxe2x80x9d refer to coals having a total iron content of at least about 10 wt. % (dry basis of the ash) while xe2x80x9clow iron coalsxe2x80x9d refer to coals having a total iron content of less than about 10 wt. % (dry basis of the ash). As will be appreciated, iron and sulfur are typically present in coal in the form of ferrous or ferric carbonites and/or sulfides, such as iron pyrite.
The transition from high sulfur (and high iron) to low sulfur (and low iron) coals has created many problems for slag-type coal furnaces such as cyclone furnaces. When low-sulfur western coals, with low iron and high (i.e., at least about 20 wt. % (dry basis of the ash)) alkali (e.g., calcium) contents, are fired in these boilers, the viscosity of the slag is too low, causing less retained bottom ash (or a higher amount of entrained coal and ash particulates in the offgas from combustion), degraded performance of particulate collectors (due to the increased particulate load) and therefore a higher incidence of stack opacity violations and increased fuel and maintenance costs, less reliable slag tapping, the occurrence of flames in the main furnace, high furnace exit temperatures (or sprays), and increased convective pass fouling. As shown in FIG. 3, in the operating range noted above high sulfur coals (denoted as Illinois coal) form slag having a moderate to high viscosity and therefore produce a relatively thick slag layer on the surface of the furnace while low sulfur coals (denoted as PRB coals) form a slag having a very low viscosity and therefore produce thin, low viscosity slag layers. As a result, utilities using slag-type furnaces, such as cyclone furnaces, have, through switching feed materials, realized lower sulfur dioxide emissions but at the same time have produced a host of new operational problems.
Techniques that have been employed to provide improved slag characteristics for high sulfur eastern coals have proven largely ineffective for low sulfur coals. For example, limestone has been used by utilities as a high sulfur coal additive to adjust the slag viscosity to the desired range for the furnace operating temperature. The calcium in the limestone is widely believed to be the primary reason for the improved performance. Low sulfur western coals, in contrast, already have relatively high calcium contents and therefore experience little, if any, viscosity adjustment when limestone is added to the coal feed to the furnace.
Another possible solution is the addition of iron pellets (which typically include at least predominantly nonoxidized iron) to the furnace to assist in slag formation and coal combustion. Iron oxide fluxes high-silica glass, while reduced forms of iron (FeO or Fe-metal) flux calcium-rich glass. In the presence of burning coal particles, iron exists primary in reduced form. The use of iron has been recommended to solve slag-tapping problems in cyclone furnaces by adding commercially available iron pellets, which are very expensive. The pellets have a further disadvantage of forming pools of reduced iron that can be very corrosive to metal or refractory surfaces exposed to the iron and/or of being an ineffective fluxing agent. Therefore, iron fluxes have failed to achieve long term acceptance in the utility industry.
Another possible solution is to blend high iron coals with the western coals to increase the iron content of the coal feed. Blended coals are far from a perfect solution. High iron coals (or xe2x80x9ckickerxe2x80x9d coals) are often much more expensive coals than western coals. High iron coals also have high sulfur levels because the predominant form of iron in such coals is iron sulfide (or iron pyrite). Blended coals suffer from increased operating costs and increased sulfur dioxide emissions, which can in certain cases exceed applicable regulations.
Another possible solution is to grind the coal going into the cyclone furnace much finer and supply additional air to increase the percentage of combustion that occurs for coal particles in flight. This option requires expensive modifications or replacement of grinding equipment and is counter to the original design and intent of the cyclone furnace. The technique further decreases boiler efficiency and increases the auxiliary power required to operate the boiler. The use of fine grinding has thus proven to be an inadequate solution to the problem in most cases.
The various methods and compositions of the present invention can provide a fluxing agent or additive that can be contacted with the coal feed to or in a combustion chamber of a furnace to produce a slag layer having one or more desirable characteristics, such as viscosity and thickness. The methods and compositions are particularly effective for a cyclone furnace of the type illustrated in FIG. 1.
In one embodiment, a method is provided for combusting coal that includes the steps of:
(a) providing a coal-containing feed material to a coal combustion chamber;
(b) contacting the feed material with an iron-containing additive; and
(c) melting at least a portion of the coal-containing feed material and iron-containing additive to form a slag layer on at least a portion of a surface of the combustion chamber, whereby coal in the coal-containing feed material is captured by the slag layer and combusted. As noted below, the additive permits slag-type furnaces to burn low iron, high alkali, and low sulfur coals by enhancing the slagging characteristics of the ash.
The coal-containing feed material has coal as the primary component. As used herein, xe2x80x9ccoalxe2x80x9d refers to macromolecular network comprised of groups of polynuclear aromatic rings, to which are attached subordinate rings connected by oxygen, sulfur and aliphatic bridges. Coal comes in various grades including peat, lignite, sub-bituminous coal and bituminous coal. In one process configuration, the coal includes less than about 1.5 wt. % (dry basis of the coal) sulfur while the coal ash contains less than about 10 wt. % (dry basis of the ash) iron as Fe2O3, and at least about 15 wt. % calcium as CaO (dry basis of the ash). The material is preferably in the form of a free flowing particulate having a P90 size of no more than about 0.25 inch.
The coal combustion chamber is part of a coal-fired furnace. The furnace can be of any configuration, with a slag-type furnace being preferred and a cyclone furnace being even more preferred.
The iron-containing additive can be in any form and any composition so long as iron is present in sufficient amounts to flux effectively the feed material. The iron can be present in any form(s) that fluxes under the conditions of the furnace, including in the forms of ferrous or ferric oxides and sulfides. In one formulation, iron is present in the form of both ferric and ferrous iron, with ferric and ferrous iron oxides being preferred. Preferably, the ratio of ferric (or higher valence) iron to ferrous (or lower valence) iron is less than 2:1 and more preferably ranges from about 0.1:1 to about 1.95:1, or more preferably at least about 33.5% of the iron in the additive is in the form of ferrous (or lower valence) iron and no more than about 66.5% of the iron in the additive is in the form of ferric (or higher valence) iron. In a particularly preferred formulation, at least about 10% of the iron in the additive is in the form of wustite. xe2x80x9cWustitexe2x80x9d refers to the oxide of iron of low valence which exist over a wide range of compositions (e.g. that may include the stoichiometric composition FeO) as compared to xe2x80x9cmagnetitexe2x80x9d which refers to the oxide of iron of intermediate or high valence which has a stoichiometric composition of Fe2O3 (or FeO.Fe2O3). It has been discovered that the additive is particularly effective when wustite is present in the additive. While not wishing to be bound by any theory, it is believed that the presence of iron of low valence levels (e.g., having a valence of 2 or less) in oxide form may be the reason for the surprising and unexpected effectiveness of this additive composition.
While not wishing to be bound by any theory, it is believed that the presence of iron in the calcium aluminosilicate slags of western coals causes a decrease in the melting temperature of the ash and crystal formation in the melt when a critical temperature (TCV) is reached. These crystals change the flow characteristics of the slag causing the slag to thicken before the slag can flow. This phenomenon is known as xe2x80x9cyield stressxe2x80x9d and is familiar to those skilled in the art of non-Newtonian flow. Thicker slag allows the slag to capture and hold more coal particles. Therefore, fewer coal particles escape the combustor without being burned.
In a preferred process configuration, the additive is in the form of a free-flowing particulate having a P90 size of no more than about 300 microns (0.01 inch) and includes at least about 50 wt. % iron. Compared to iron pellets, the relatively small particle size of the additive reduces significantly the likelihood of the formation of pools of reduced iron that can be very corrosive to metal or refractory surfaces exposed to the iron. It is believed that the reason for pooling and poor fluxing has been the relatively large sizes of iron pellets (typically the P90 size of the pellets is at least about 0.25 inch (6350 microns)) in view of the short residence times of the pellets in the combustion chamber. Such pellets take longer to heat and therefore melt and act as a flux. This can cause the pellets to pass or tumble through the chamber before melting has fully occurred. The increase surface area of the additive further aids in more effective fluxing as more additive reaction surface is provided.
Preferably, the additive further includes a mineralizer, such as zinc oxide. While not wishing to be bound by any theory, it is believed that the zinc increases the rate at which iron fluxes with the coal ash. xe2x80x9cAshxe2x80x9d refers to the residue remaining after complete combustion of the coal particles. Ash typically includes mineral matter (silica, alumina, iron oxide, etc.) Zinc is believed to act as a mineralizer. Mineralizers are substances that reduce the temperature at which a material sinters by forming solid solutions. This is especially important where, as here, the coal/ash residence time in the combustor is extremely short (typically less than about one second). Preferably, the additive includes at least about 1 wt. % (dry basis) mineralizer and more preferably, the additive includes from about 3 to about 5 wt. % (dry basis) mineralizer. Mineralizers other than zinc oxides include calcium, magnesium or manganese flourides or sulfites and other compounds known to those in the art of cement-making. Preferably, the additive includes no more than about 0.5 wt. % (dry basis) sulfur, more preferably includes no more than about 0.1 wt. % (dry basis) sulfur, and even more preferably is at least substantially free of sulfur.
The injection rate of the iron-containing additive to the chamber depends, of course, on the combustion conditions and the chemical composition of the coal feed and additive. Typically, the injection rate of the iron-containing additive into the combustion chamber ranges from about 10 to about 50 lb/ton coal and more typically from about 10 to about 20 lb/ton coal.
After combination with the additive, the coal-containing feed material typically includes:
(a) coal; and
(b) an additive that includes iron in an amount of at least about 0.5 wt. % (dry basis) and mineralizer in an amount of at least about 0.005 wt. % (dry basis).
The methods and additives of the present invention can have a number of advantages compared to conventional systems. The additive(s) can provide a slag layer in the furnace having the desired viscosity and thickness at a lower operation temperature. As a result, there is more bottom ash to sell, a relatively low flyash carbon content, more effective combustion of the coal, more reliable slag tapping, improved boiler heat transfer, and a relatively low amount of entrained particulates in the offgas from combustion, leading to little or no degradation in performance of particulate collectors (due to the increased particulate load). The boiler can operate at lower power loads (e.g., 60 MW without the additive and only 35 MW with the additive as set forth below) without freezing the slag tap and risking boiler shutdown. The operation of the boiler at a lower load (and more efficient units can operate at higher load) when the price of electricity is below the marginal cost of generating electricity, can save on fuel costs. The additive can reduce the occurrence of flames in the main furnace, lower furnace exit temperatures (or steam temperatures), and decrease the incidence of convective pass fouling compared to existing systems. The additive can have little, if any, sulfur, thereby not adversely impacting sulfur dioxide emissions. These and other advantages will become evident from the following discussion.